Depiction of Celts
The Celts are portrayed as autarkic people who make their living on cattle breeding and hunting. They are very capable horsemen who can cull wild boar from a horseback. Besides that they inhabit villages that are fortified by palisades. (In "The Pupil" and "Daughter of the King" the outside of Arthur's Celtic village is shown.) That is how they are defined as Celts.
Read more about this topic: Arthur Of The Britons